Gonna Be The Twin Tails!!

Gonna Be The Twin Tails!! is a Japanese light novel series. Soji Mitsuka is an ordinary high school boy who has an obsession for twintail hair. One day he encounters a mysterious girl named Twoearle, who is from a parallel world, when monsters appear in his town and declare that all twintails in the world belong to them. These monsters feed off "attribute strength", the spiritual energy of humans. Twoearle entrusted Soji with Tail Gear, imaginary armor initiated by powerful twintail attributes. With the armor, Soji transforms into a girl and twintail warrior (ツインテイルズ関係者, tsuinteiruzu kankei-sha) branded Tail Red, to protect Earth.